The present invention relates to encoders for sensing the position of a first member moving relative to a second member. More particularly, the present invention relates to a sensor providing an absolute position of the moving member.
A number of absolute position encoders have developed. These position encoders include optical encoders wherein a decoder track is moved between a source of light and a photo detector so that the position of the track relative to the photodetector can be determined. Other optical arrangements employ tracks from which a binary word can be read at a position along the track, with the word uniquely defining a position along the length of the track.
Magnetic sensor devices have also been employed to detect absolute position. The fixed member includes multiple channels of magnetic material arranged in a pattern such as a binary code or a Gray code. Magnetoresistive resistors, which increase their resistance in the presence of a magnetic field, are located on the moving member. The potential across the magnetoresistive resistors is representative of the absolute position of the moving member, since the encoded pattern of the magnet provides a unique amount of resistance change. The smallest distance which such as position encoder can resolve is limited to the smallest spacing between the codes. These same types of position encoders are also suspectible to installation errors. In particular, the fixed and the moving members must be parallel to each other over the travel of the moving member so that the magnetic field is consent. The requirement that the magnetic field be constant limits the interchangeability of the members, in that the position encoder provides improper output if the dimensions of a replacement part are different from the part that was replace. In other words, such position encoders must be re-aligned when either the members to which the source or the encoder is affixed is replaced.
Therefore, there is a need for an absolute position encoder which works without elaborate alignment procedures, promotes interchangeability of components during field repairs and has an improved resolution over existing encoders.